The mains gases and coke oven gases that were obtained via thermal processes were used for a long time in the past as gaseous fuels, both for public lighting and for domestic needs. These gases contained strongly odoriferous components. They consequently had a strong intrinsic odor, enabling a gas leak to be easily detected.
In contrast, the gaseous fuels used nowadays, whether natural gas, propane, butane, liquefied petroleum gas (or LPG), or even oxygen (for example for welding), are essentially odorless, either on account of their origin or on account of the purification treatment they have received.
Thus, if leaks are not perceived in time, explosable mixtures of gaseous fuels and air rapidly form, with a consequential high risk potential.
It is for these safety reasons that the natural gas circulating in gas pipelines is odorized by injection (in specialized plants) of suitable additives known as odorizers.
Natural gas is generally conveyed in odorless form to consumer countries from the production sites, after a suitable purification treatment, either via a gas pipeline, or (in liquid form) in specialized ships (methane tankers). In France, for example, natural gas is thus received at a limited number of injection plants, where the odorizer is injected, such that the natural gas both which circulates in the French gas pipeline network and which is stored in underground reservoirs is odorized, thus allowing easy detection in the event of a leak, irrespective of the portion of the network in which this leak occurs.
In other countries, natural gas may be distributed throughout the territory via a network of gas pipelines in which it circulates without an odorizer, the gas then being odorized on entering the towns where it is consumed, hence the need for an even larger number of injection plants.
Storage tanks are usually maintained under an atmosphere of nitrogen or of natural gas in order to limit, at this stage, the risks of explosion.
It is known practice to use alkyl sulfides and/or mercaptans as odorizers, alone or as a mixture. Examples that may be mentioned include diethyl sulfide, dimethyl sulfide, methyl ethyl sulfide, tetrahydrothiophene, tert-butyl mercaptan and isopropyl mercaptan, which are widely used for their excellent properties, being especially capable of triggering a sensation of alarm among people in the event of an accidental leak of the natural gas thus odorized, and of initiating the necessary safety operations within the briefest of delays.
However, during the combustion of natural gas, these products generate an amount of sulfur dioxide which, small as it may be, becomes non-negligible when an overall account is taken at the scale of a country or a region, especially one with a high level of industrialization or of urbanization. Thus, for example, the combustion of a natural gas odorized with tetrahydrothiophene (THT) at a concentration of 10 mg/Nm3 (or number of m3 of gas, measured under standard temperature and pressure conditions) generates 7.3 mg/Nm3 of sulfur dioxide.
In the general context of better assimilation of environmental constraints, it is thus necessary to reduce the amounts of SO2 discharged into the atmosphere via the odorizers present in natural gas, during the combustion thereof.
Many odorizing mixtures free of sulfur compounds have been proposed.
Examples that may be mentioned include PL 72057, which describes odorizing mixtures based on dicyclopentadiene, JP-41-73895, which describes mixtures of specific ethers and esters, WO 02/42396, which describes mixtures based on norbornene or derivatives thereof, and JP-80-060 167, which describes mixtures based on 5-ethylidene-2-norbornene and a 2-alkoxy-3-alkylpyrazine.
Many documents describing odorizing mixtures based on alkyl acrylates are also known:
JP-49-131 201 describes a gaseous fuel odorizer based on an acrylate CH2═CHCO2—R1 where R1 is a saturated or unsaturated hydrocarbon-based chain containing 3 carbon atoms and/or based on an ether R2—O—R3 where R2 is an unsaturated hydrocarbon-based chain containing 2 or 3 carbon atoms and R3 is a saturated or unsaturated hydrocarbon-based chain containing 2 or 3 carbon atoms.
DE 198 37 066 describes a process for odorizing natural gas by adding a mixture comprising an alkyl acrylate, a nitrogenous compound of pyrazine type, and an antioxidant. However, this mixture has the drawback of not having a characteristic odor of gas and is thus liable to lead to confusion in the event of a gas leak. The risk is, quite obviously, that of not detecting this leak, and thus of explosion, if the concentration of gas in the air reaches its lower explosiveness limit.
Documents are also known that combine sulfur compound(s) and non-sulfur compound(s), such as JP-55-137 190, which describes an odorizing mixture combining ethyl acrylate with a specific sulfur compound, namely tert-butyl mercaptan (or TBM). However, the major drawback of this mixture is that, on account of the chemical reactivity of TBM with ethyl acrylate, the two components of the odorizing mixture must be stored, at the various injection plants, in separate tanks and also require separate pumps and injection heads for introduction into the gas pipeline. With regard to the complex logistics for odorizing natural gas presented hereinabove, this results in a considerable increase in costs for the injection plants, arising from the necessary multiplication of the storage tanks, pumps and injection heads; WO 2004/024 852 describes an odorizer constituted of four components, including an alkyl acrylate, an alkyl sulfide and a stabilizing antioxidant such as tert-butylhydroxytoluene, hydroquinone, etc.; WO 2005/103 210 describes an odorizing mixture for odorless gaseous fuel, constituted of an alkyl sulfide, an alkyl acrylate and a compound for inhibiting the polymerization of the alkyl acrylate, of nitroxide type.
It is known that acrylates are highly reactive monomers that can polymerize spontaneously, especially on storage, to form polyacrylates. Such an uncontrolled polymerization is liable to place in danger people located in proximity to the injection plants, such as local residents or maintenance workers, due to the risk of explosion. This polymerization arising during storage, including, for example, in the storage tanks or vats of the injection plants, may also lead to rapid fouling or even blocking of the pipes between the storage tanks and the point of injection. Such a phenomenon may lead to an uncontrolled drop in the concentration of odorizer in the natural gas, which increases the risk associated with an undetected gas leak.
To avoid this, hydroquinones are commonly added to acrylate-based odorizing compositions to inhibit their polymerization, as taught in U.S. Pat. No. 3,816,627, which concerns the manufacture of acrylate. In order to function, the hydroquinone-based inhibiting system needs oxygen, since the active form of the inhibitor is a molecule comprising a radical that is formed following the reaction of the inhibitor with oxygen and traps the polymerization precursors. This inhibitor requires storage of the odorizing mixture in air. This condition is not respected when an odorizer storage tank is under pressure of natural gas, which makes it possible to increase the yield of the pumps for injecting odorizer into the gas. Storage under nitrogen also exists. In this case, as with natural gas, the hydroquinone cannot react with oxygen to form a radical and therefore does not play its role of inhibitor, which places the user in danger of risk of explosion following an uncontrolled polymerization, but also may cause fouling or even rapid blocking of the pipes between the storage reservoir and the point of injection. The consequence of this latter point is an uncontrolled drop in the concentration of odorizer in the gas, leading to an increased risk of explosion due to undetected gas leaks.